Lithium ion battery anodes including graphenic carbon particles

ABSTRACT

Lithium ion battery anodes including graphenic carbon particles are disclosed. Lithium ion batteries containing such anodes are also disclosed. The anodes include mixtures of lithium-reactive metal particles such as silicon, graphenic carbon particles, and a binder. The use of graphenic carbon particles in the anodes results in improved performance of the lithium ion batteries.

FIELD OF THE INVENTION

The present invention relates to lithium ion battery electrodesincluding graphenic carbon particles.

BACKGROUND OF THE INVENTION

Lithium ion batteries are well known. Silicon has been proposed for useas an active material for lithium ion batteries due to its very largetheoretical specific capacity, which is more than an order of magnitudegreater than the theoretical capacity of commonly used commercial carbonanodes. Tin has also been proposed for use as an active material due toits large specific capacity. A problem with these materials is that alarge expansion takes place when they store lithium, which can result infracturing and pulverization during charge-discharge cycling of thebatteries. Capacity retention is therefore poor since the fractured andfragmented active material loses electrical contact with the batteryanodes.

SUMMARY OF THE INVENTION

An aspect of the invention provides a lithium ion battery anode materialcomprising lithium-reactive metal particles, graphenic carbon particles,and a binder.

Another aspect of the invention provides a lithium ion batterycomprising an anode, a cathode, a separator between the anode and thecathode, and an electrolyte in contact with the anode and the cathode,wherein the anode comprises lithium-reactive metal particles, grapheniccarbon particles, and a binder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic side sectional view of a lithium ionbattery including an anode comprising graphenic carbon particles inaccordance with an embodiment of the present invention.

FIGS. 2 and 3 are graphs of specific capacity versus cycle numbers forvarious test batteries.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 schematically illustrates a lithium ion battery 100 in accordancewith an embodiment of the present invention. The battery 100 includes ananode 20, a cathode 10, a separator 40 between the anode and cathode,and an electrolyte 30 in contact with the anode and cathode. A casing 50is provided in electrical contact with the anode 20. A terminal 60 is inelectrical contact with the cathode 10.

The cathode 10 may be made of any known conductive materialconventionally used in lithium ion batteries, such as copper or othermetals. The electrolyte 30 may comprise any known electrolyte materialconventionally used in lithium ion batteries, such as lithium-containingelectrolyte salts dissolved in organic solvents. Examples oflithium-containing electrolyte salts include LiClO₄, LiAsF₆, LiPF₆,LiBF₄, LiB(C₆H₅)₄, LiB(C₂O₄)₂, CH₃SO₃Li, CF₃SO₃Li, LiCl, LiBr and thelike. Examples of organic solvents include propylene carbonate, ethylenecarbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane,1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether,sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole,acetate, butyrate, propionate and the like. In certain embodiments,cyclic carbonates such as propylene carbonate, or chain carbonates suchas dimethyl carbonate and diethyl carbonate may be used. These organicsolvents can be used singly or in a combination of two types or more. Incertain embodiments, the electrolyte 30 may also comprise additives orstabilisers such as VC (vinyl carbonate), VEC (vinyl ethylenecarbonate), EA (ethylene acetate), TPP (triphenylphosphate),phosphazenes, LiBOB, LiBETI, LiTFSI, BP (biphenyl), PS (propylenesulfite), ES (ethylene sulfite), AMC (allylmethylcarbonate), and APV(divinyladipate).

In accordance with embodiments of the invention, the anode comprises aconductive substrate such as copper foil, or other metal foils, having agraphenic carbon particle-containing coating of the present inventiondeposited on one or both sides thereof. The graphenic carbonparticle-containing anode material may include a mixture of thegraphenic carbon particles with lithium-reactive particles such as Siand/or Sn and a binder.

In accordance with certain embodiments, the anode material comprisesfrom 15 to 85 weight percent lithium-reactive metal particles, from 3 to75 weight percent graphenic carbon particles, and from 3 to 60 weightpercent binder. For example, the lithium-reactive metal particles maycomprise from 25 to 70 weight percent, or from 30 to 50 weight percent.In certain embodiments, the graphenic carbon particles may comprise from10 to 60 weight percent, or from 30 to 50 weight percent.

In certain embodiments, the lithium-reactive metal particles compriseSi, Sn or a combination thereof. The lithium-reactive metal particlesmay typically have an average particle size of less than 1,000nanometers, for example, from 5 to 200 nanometers, or from 10 to 120nanometers.

In certain embodiments, the binder of the anode material comprises apolymer. For example, the polymeric binder may include poly(acrylicacid) (PAA), acrylate polymers containing greater than 5 weight percentacrylic acid, carboxymethylcellulose, polymethacrylic acid, acrylatepolymers containing greater than 5 weight percent methacrylic acid,polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), acryliclatex dispersions, and the like.

The graphenic carbon particles used in the anodes of the presentinvention may be obtained from commercial sources, for example, fromAngstron, XG Sciences and other commercial sources. In certainembodiments discussed in detail below, the graphenic carbon particlesmay be thermally produced in accordance with the methods and apparatusdescribed in U.S. application Ser. Nos. 13/249,315 and 13/309,894, whichare incorporated herein by reference.

As used herein, the term “graphenic carbon particles” means carbonparticles having structures comprising one or more layers ofone-atom-thick planar sheets of sp²-bonded carbon atoms that are denselypacked in a honeycomb crystal lattice. The average number of stackedlayers may be less than 100, for example, less than 50. In certainembodiments, the average number of stacked layers is 30 or less, such as20 or less, 10 or less, or, in some cases, 5 or less. The grapheniccarbon particles may be substantially flat, however, at least a portionof the planar sheets may be substantially curved, curled, creased orbuckled. The particles typically do not have a spheroidal or equiaxedmorphology.

In certain embodiments, the graphenic carbon particles present in theanode compositions of the present invention have a thickness, measuredin a direction perpendicular to the carbon atom layers, of no more than10 nanometers, no more than 5 nanometers, or, in certain embodiments, nomore than 4 or 3 or 2 or 1 nanometers, such as no more than 3.6nanometers. In certain embodiments, the graphenic carbon particles maybe from 1 atom layer up to 3, 6, 9, 12, 20 or 30 atom layers thick, ormore. In certain embodiments, the graphenic carbon particles present inthe compositions of the present invention have a width and length,measured in a direction parallel to the carbon atoms layers, of at least50 nanometers, such as more than 100 nanometers, in some cases more than100 nanometers up to 500 nanometers, or more than 100 nanometers up to200 nanometers. The graphenic carbon particles may be provided in theform of ultrathin flakes, platelets or sheets having relatively highaspect ratios (aspect ratio being defined as the ratio of the longestdimension of a particle to the shortest dimension of the particle) ofgreater than 3:1, such as greater than 10:1.

In certain embodiments, the graphenic carbon particles used in the anodecompositions of the present invention have relatively low oxygencontent. For example, the graphenic carbon particles used in certainembodiments of the compositions of the present invention may, even whenhaving a thickness of no more than 5 or no more than 2 nanometers, havean oxygen content of no more than 2 atomic weight percent, such as nomore than 1.5 or 1 atomic weight percent, or no more than 0.6 atomicweight, such as about 0.5 atomic weight percent. The oxygen content ofthe graphenic carbon particles can be determined using X-rayPhotoelectron Spectroscopy, such as is described in D. R. Dreyer et al.,Chem. Soc. Rev. 39, 228-240 (2010).

In certain embodiments, the graphenic carbon particles used in the anodecompositions of the present invention have a B.E.T. specific surfacearea of at least 50 square meters per gram, such as 70 to 1000 squaremeters per gram, or, in some cases, 200 to 1000 square meters per gramsor 200 to 400 square meters per gram. As used herein, the term “B.E.T.specific surface area” refers to a specific surface area determined bynitrogen adsorption according to the ASTMD 3663-78 standard based on theBrunauer-Emmett-Teller method described in the periodical “The Journalof the American Chemical Society”, 60, 309 (1938).

In certain embodiments, the graphenic carbon particles used in the anodecompositions of the present invention have a Raman spectroscopy 2D/Gpeak ratio of at least 1:1, for example, at least 1.2:1 or 1.3:1. Asused herein, the term “2D/G peak ratio” refers to the ratio of theintensity of the 2D peak at 2692 cm⁻¹ to the intensity of the G peak at1,580 cm⁻¹.

In certain embodiments, the graphenic carbon particles used in the anodecompositions of the present invention have a relatively low bulkdensity. For example, the graphenic carbon particles used in certainembodiments of the present invention are characterized by having a bulkdensity (tap density) of less than 0.2 g/cm³, such as no more than 0.1g/cm³. For the purposes of the present invention, the bulk density ofthe graphenic carbon particles is determined by placing 0.4 grams of thegraphenic carbon particles in a glass measuring cylinder having areadable scale. The cylinder is raised approximately one-inch and tapped100 times, by striking the base of the cylinder onto a hard surface, toallow the graphenic carbon particles to settle within the cylinder. Thevolume of the particles is then measured, and the bulk density iscalculated by dividing 0.4 grams by the measured volume, wherein thebulk density is expressed in terms of g/cm³.

In certain embodiments, the graphenic carbon particles used in the anodecompositions of the present invention have a compressed density and apercent densification that is less than the compressed density andpercent densification of graphite powder and certain types ofsubstantially flat graphenic carbon particles. Lower compressed densityand lower percent densification are each currently believed tocontribute to better dispersion and/or rheological properties thangraphenic carbon particles exhibiting higher compressed density andhigher percent densification. In certain embodiments, the compresseddensity of the graphenic carbon particles is 0.9 or less, such as lessthan 0.8, less than 0.7, such as from 0.6 to 0.7. In certainembodiments, the percent densification of the graphenic carbon particlesis less than 40%, such as less than 30%, such as from 25 to 30%.

For purposes of the present invention, the compressed density ofgraphenic carbon particles is calculated from a measured thickness of agiven mass of the particles after compression. Specifically, themeasured thickness is determined by subjecting 0.1 grams of thegraphenic carbon particles to cold press under 15,000 pound of force ina 1.3 centimeter die for 45 minutes, wherein the contact pressure is 500MPa. The compressed density of the graphenic carbon particles is thencalculated from this measured thickness according to the followingequation:

${{Compressed}\mspace{14mu} {Density}\mspace{14mu} \left( {g\text{/}{cm}^{3}} \right)} = \frac{0.1\mspace{14mu} {grams}}{\Pi*\left( {1.3\mspace{14mu} {cm}\text{/}2} \right)^{2}*\left( {{measured}\mspace{14mu} {thickness}\mspace{14mu} {in}\mspace{14mu} {cm}} \right)}$

The percent densification of the graphenic carbon particles is thendetermined as the ratio of the calculated compressed density of thegraphenic carbon particles, as determined above, to 2.2 g/cm³, which isthe density of graphite.

In certain embodiments, the graphenic carbon particles have a measuredbulk liquid conductivity of at least 100 microSiemens, such as at least120 microSiemens, such as at least 140 microSiemens immediately aftermixing and at later points in time, such as at 10 minutes, or 20minutes, or 30 minutes, or 40 minutes. For the purposes of the presentinvention, the bulk liquid conductivity of the graphenic carbonparticles is determined as follows. First, a sample comprising a 0.5%solution of graphenic carbon particles in butyl cellosolve is sonicatedfor 30 minutes with a bath sonicator. Immediately following sonication,the sample is placed in a standard calibrated electrolytic conductivitycell (K=1). A Fisher Scientific AB 30 conductivity meter is introducedto the sample to measure the conductivity of the sample. Theconductivity is plotted over the course of about 40 minutes.

In accordance with certain embodiments, percolation, defined as longrange interconnectivity, occurs between the conductive graphenic carbonparticles. Such percolation may reduce the resistivity of the coatingcompositions. The conductive graphenic particles may occupy a minimumvolume within the coating such that the particles form a continuous, ornearly continuous, network. In such a case, the aspect ratios of thegraphenic carbon particles may affect the minimum volume required forpercolation. Furthermore, the surface energy of the graphenic carbonparticles may be the same or similar to the surface energy of theelastomeric rubber. Otherwise, the particles may tend to flocculate ordemix as they are processed.

The graphenic carbon particles utilized in the anode compositions of thepresent invention can be made, for example, by thermal processes. Inaccordance with embodiments of the invention, thermally-producedgraphenic carbon particles are made from carbon-containing precursormaterials that are heated to high temperatures in a thermal zone such asa plasma. The carbon-containing precursor, such as a hydrocarbonprovided in gaseous or liquid form, is heated in the thermal zone toproduce the graphenic carbon particles in the thermal zone or downstreamtherefrom. For example, thermally-produced graphenic carbon particlesmay be made by the systems and methods disclosed in U.S. patentapplication Ser. Nos. 13/249,315 and 13/309,894.

In certain embodiments, the graphenic carbon particles may be made byusing the apparatus and method described in U.S. patent application Ser.No. 13/249,315 at [0022] to [0048] in which (i) one or more hydrocarbonprecursor materials capable of forming a two-carbon fragment species(such as n-propanol, ethane, ethylene, acetylene, vinyl chloride,1,2-dichloroethane, allyl alcohol, propionaldehyde, and/or vinylbromide) is introduced into a thermal zone (such as a plasma), and (ii)the hydrocarbon is heated in the thermal zone to a temperature of atleast 1,000° C. to form the graphenic carbon particles. In otherembodiments, the graphenic carbon particles may be made by using theapparatus and method described in U.S. patent application Ser. No.13/309,894 at [0015] to [0042] in which (i) a methane precursor material(such as a material comprising at least 50 percent methane, or, in somecases, gaseous or liquid methane of at least 95 or 99 percent purity orhigher) is introduced into a thermal zone (such as a plasma), and (ii)the methane precursor is heated in the thermal zone to form thegraphenic carbon particles. Such methods can produce graphenic carbonparticles having at least some, in some cases all, of thecharacteristics described above.

During production of the graphenic carbon particles by the thermalproduction methods described above, a carbon-containing precursor isprovided as a feed material that may be contacted with an inert carriergas. The carbon-containing precursor material may be heated in a thermalzone, for example, by a plasma system. In certain embodiments, theprecursor material is heated to a temperature ranging from 1,000° C. to20,000° C., such as 1,200° C. to 10,000° C. For example, the temperatureof the thermal zone may range from 1,500 to 8,000° C., such as from2,000 to 5,000° C. Although the thermal zone may be generated by aplasma system, it is to be understood that any other suitable heatingsystem may be used to create the thermal zone, such as various types offurnaces including electrically heated tube furnaces and the like.

The gaseous stream may be contacted with one or more quench streams thatare injected into the plasma chamber through at least one quench streaminjection port. The quench stream may cool the gaseous stream tofacilitate the formation or control the particle size or morphology ofthe graphenic carbon particles. In certain embodiments of the invention,after contacting the gaseous product stream with the quench streams, theultrafine particles may be passed through a converging member. After thegraphenic carbon particles exit the plasma system, they may becollected. Any suitable means may be used to separate the grapheniccarbon particles from the gas flow, such as, for example, a bag filter,cyclone separator or deposition on a substrate.

Without being bound by any theory, it is currently believed that theforegoing methods of manufacturing graphenic carbon particles areparticularly suitable for producing graphenic carbon particles havingrelatively low thickness and relatively high aspect ratio in combinationwith relatively low oxygen content, as described above. Moreover, suchmethods are currently believed to produce a substantial amount ofgraphenic carbon particles having a substantially curved, curled,creased or buckled morphology (referred to herein as a “3D” morphology),as opposed to producing predominantly particles having a substantiallytwo-dimensional (or flat) morphology. This characteristic is believed tobe reflected in the previously described compressed densitycharacteristics and is believed to be beneficial in the presentinvention because, it is currently believed, when a significant portionof the graphenic carbon particles have a 3D morphology, “edge to edge”and “edge-to-face” contact between graphenic carbon particles within thecomposition may be promoted. This is thought to be because particleshaving a 3D morphology are less likely to be aggregated in thecomposition (due to lower Van der Waals forces) than particles having atwo-dimensional morphology. Moreover, it is currently believed that evenin the case of “face to face” contact between the particles having a 3Dmorphology, since the particles may have more than one facial plane, theentire particle surface is not engaged in a single “face to face”interaction with another single particle, but instead can participate ininteractions with other particles, including other “face to face”interactions, in other planes. As a result, graphenic carbon particleshaving a 3D morphology are currently thought to provide the bestconductive pathway in the present compositions and are currently thoughtto be useful for obtaining electrical conductivity characteristicssought by embodiments of the present invention.

The following examples are intended to illustrate various aspects of theinvention, and are not intended to limit the scope of the invention.

EXAMPLES

Anode materials comprising mixtures of silicon particles and differenttypes of graphenic carbon particles or carbon black particles in apolymeric binder were made. The graphenic carbon particles used inSamples A and B were produced by a thermal-production method utilizingmethane as a precursor material disclosed in U.S. patent applicationSer. No. 13/309,894. The thermally-produced graphenic carbon particlesof Sample A were further treated with a toluene solution to extract anyresidual low molecular weight hydrocarbon contaminants. The graphenicparticles used in Sample C were XG300 particles commercially availablefrom XG Sciences. The carbon black particles used in Sample D werecommercially available Super P carbon black particles.

Electrochemical experiments were performed on Samples A-D using2016-type coin cells, which were assembled in an argon-filled dryglovebox (MBraun, Inc.) with the Si electrode as the working electrodeand the Li metal as the counter electrode. The working electrodes wereprepared by casting a slurry consisting of 40 weight percent Siparticles (50 nm nanoparticles from Sigma), 40 weight percent graphenicparticles or carbon black particles, and 20 weight percent poly(acrylicacid) (PAA) binder. 1 mol L-1 LiPF₆ in a mixture of ethylene carbonate,diethyl carbonate and dimethyl carbonate (EC: DEC: DMC, 2:1:2 by vol. %)and 10 weight percent fluoroethylene carbonate (FEC) was used as theelectrolyte (Novolyte Technologies, Independence, Ohio). Electrochemicalperformance was evaluated by galvanostatic charge/discharge cycling onan Arbin BT-2000 battery tester at room temperature under differentcurrent densities in the voltage range between 1.5 and 0.01 V versusLi+/Li. The current density and specific capacity are calculated basedon the mass of Si only.

Testing protocols included rate testing as follows: first 7 cyclestested using a current density of 1 A/g; current density of 2 A/g wasused afterwards.

FIG. 2 illustrates electrochemical performance of the Sample A-Dmaterials containing different types of graphenic carbon particles orcarbon black particles under various constant-current testing protocols.Based on the results shown in FIG. 2, it is clear that both Samples Aand B containing the thermally-produced graphenic particles exhibitbetter capacity retention than Samples C and D containing thecommercially available graphene and carbon black, respectively. Sample Aalso shows higher specific capacity than Sample C.

Testing protocols for the data shown in FIG. 3 are as follows: for thecapacity limited to 1600 mAh/g, a constant current of 1 A/g was used forboth of lithiation (discharge) and delithiation (charge) processes; forthe capacity limited to 3000 mAh/g, a constant current of 400 mA/g wasused for lithiation (discharge) process while a constant current of 1A/g was used for delithiation (charge) process to mimic the realapplication of anode materials in a full battery.

FIG. 3 illustrates the electrochemical performance of Sample A under aconstant-capacity testing protocol. FIG. 3 shows that Sample A maintainsthe capacity well up to 100 cycles when tested with capacity limited to1,600 and 3,000 mAh/g, respectively.

For purposes of this detailed description, it is to be understood thatthe invention may assume various alternative variations and stepsequences, except where expressly specified to the contrary. Moreover,other than in any operating examples, or where otherwise indicated, allnumbers expressing, for example, quantities of ingredients used in thespecification and claims are to be understood as being modified in allinstances by the term “about”. Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the followingspecification and attached claims are approximations that may varydepending upon the desired properties to be obtained by the presentinvention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard variation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of “1 to 10” is intended to include all sub-ranges between (andincluding) the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

In this application, the use of the singular includes the plural andplural encompasses singular, unless specifically stated otherwise. Inaddition, in this application, the use of “or” means “and/or” unlessspecifically stated otherwise, even though “and/or” may be explicitlyused in certain instances.

It will be readily appreciated by those skilled in the art thatmodifications may be made to the invention without departing from theconcepts disclosed in the foregoing description. Such modifications areto be considered as included within the following claims unless theclaims, by their language, expressly state otherwise. Accordingly, theparticular embodiments described in detail herein are illustrative onlyand are not limiting to the scope of the invention which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

We claim:
 1. A lithium ion battery anode material comprising:lithium-reactive metal particles; graphenic carbon particles; and abinder.
 2. The lithium ion battery anode material of claim 1, comprisingfrom 15 to 85 weight percent of the lithium-reactive metal particles,from 3 to 75 weight percent of the graphenic carbon particles, and from3 to 60 weight percent of the binder.
 3. The lithium ion battery anodematerial of claim 2, wherein the lithium-reactive metal particlescomprise from 25 to 70 weight percent, and the graphenic carbonparticles comprise from 10 to 60 weight percent.
 4. The lithium ionbattery anode material of claim 2, wherein the lithium-reactive metalparticles comprise from 30 to 50 weight percent, and the grapheniccarbon particles comprise from 30 to 50 weight percent.
 5. The lithiumion battery anode material of claim 1, wherein the graphenic particlesare thermally-produced graphenic carbon particles.
 6. The lithium ionbattery anode material of claim 1, wherein the graphenic particles haveaspect ratios of greater than 5:1.
 7. The lithium ion battery anodematerial of claim 1, wherein the graphenic particles have averagethicknesses of from 0.3 to 6 nanometers.
 8. The lithium ion batteryanode material of claim 1, wherein the lithium-reactive metal comprisesSi, Sn or a combination thereof.
 9. The lithium ion battery anodematerial of claim 1, wherein the lithium-reactive metal particlescomprise Si having an average particle size of less than 1,000nanometers.
 10. The lithium ion battery anode material of claim 9,wherein the Si particles have an average particle size of from 5 to 200nanometers.
 11. The lithium ion battery anode material of claim 9,wherein the Si particles have an average particle size of from 10 to 120nanometers.
 12. The lithium ion battery anode material of claim 1,wherein the binder comprises a polymer.
 13. The lithium ion batteryanode material of claim 12, wherein the polymer comprises poly(acrylicacid).
 14. The lithium ion battery anode material of claim 1, whereinthe battery anode material is provided as a layer on a conductivesubstrate.
 15. The lithium ion battery anode material of claim 14,wherein the conductive substrate comprises metal foil.
 16. The lithiumion battery anode material of claim 14, wherein the layer of batteryanode material has a thickness of from 5 to 500 microns.
 17. The lithiumion battery anode material of claim 14, wherein the layer of batteryanode material has a thickness of from 20 to 200 microns.
 18. Thelithium ion battery anode material of claim 14, wherein the batteryanode material has an electrical resistivity of less than 250ohms/square.
 19. A lithium ion battery comprising: an anode; a cathode;a separator between the anode and the cathode; and an electrolyte incontact with the anode and the cathode, wherein the anode compriseslithium-reactive metal particles, graphenic carbon particles, and abinder.
 20. The lithium ion battery of claim 19, wherein the anodecomprises from 15 to 85 weight percent of the lithium-reactive metalparticles, from 3 to 75 weight percent of the graphenic carbonparticles, and from 3 to 60 weight percent of the binder.